Yes, you can use the same credentials on any other Lemmy instance. I've done this with 4 instances so far. There may be a chance your username is taken, but if you create accounts on the larger instances you should be good since the smaller user bases of the smaller instances probably won't mean that you'll be targeted.

I believe Mastodon is a different software compared to Lemmy. Lemmy is an aggregation software of the entire Fediverse, while Mastodon is more like Twitter. My limited knowledge of Mastodon makes me think that it wouldn't be possible to see Lemmy instances, but if you sent your comment using Mastodon then there might be a chance. Hopefully someone else in the comments will sound off.

I don't think you can migrate, since if that was possible your "karma" would also be able to move with you. People have been asking for that recently, but others have been saying that the Lemmy software doesn't support that.

I think the best thing you can do is make a new account on a new instance and look back at the rest of the fediverse from that instance.

For anyone that's using Lemmy.world and wants to lessen their burden on the admins, look for a Lemmy instance closer to you (in the physical world) using this site:

https://fediverse.observer/map

Sign up on that instance, sign in on your Lemmy app, sort based on All, and you should be able to see the same content as if you were on Lemmy.world!

Edit: the above map doesn't show which instances are federated with each other (or aren't). Quick Google searches seems to indicate that there is no way to see this, so it's wise to keep your Lemmy.world account open along with your smaller Lemmy account to make sure you're seeing the instances you want to see.